Monthax is a planet in the Sabbat Worlds Cluster.
During the Sabbat Worlds Crusade it was the centre of a conflict between the forces of Chaos and a number of Imperial Guard Regiments including the Tanith First and Only, the Royal Volpone 50th "Bluebloods" and the Roane Deepers. Caught in the middle was a small Eldar force led by Lord Eon Kull, attempting to seal a Webway Gate on the planet before it was captured by Chaos.
Monthax has a humid climate and is covered by dense jungle vegetation. It has no human population but does contain a number of unique plant and animal species that have adapted to its ecosystem.